I was exploring today and found that I randomly had an xp penalty hit when I had not died. I recently too the immortal power Soul Stealer but I do not see anything in the description that would indicate the risk of a death penalty.

I think that this is not bug. Here is part from wiki (http://dragontavern.wikia.com/wiki/I...s#Soul_Stealer ) that should explain it:
There is also a drawback to this power! Once you have 10 souls, you start receiving messages that you are losing grasp of the souls you collected and might lose them. I kept gathering souls and at about 14 or 15 souls I finally lost them all. When this happened (while I was level 20), I received about a 2,000 XP Debt... So not only did I lose the ability to get potential 3,000 XP from the power (Level x 100 three times, 5 souls each time), I lost XP. Effectively this was like losing about 5,000 XP.